From the Thoracic Outlet to the Brachial Plexus: A Rare Presentation of Pediatric Ewing Sarcoma.

Ewing sarcoma of the chest wall (CWES) is a highly aggressive mesenchymal tumor, the incidence of which in children under five years of age remains extremely rare. Primary involvement of the first rib and the apex of the thorax presents specific anatomical challenges, leading to severe compressive complications at an early stage. We report the case of a four-year-and-two-month-old girl admitted for right-sided neuropathic arm pain that had been present for three months and was initially attributed to a minor trauma. The gradual onset of upper extremity muscle atrophy, motor deficit predominantly affecting the intrinsic muscles of the hand, supraclavicular swelling, and a nocturnal febrile syndrome prompted a cross-sectional imaging workup. A cervicothoracoabdominopelvic CT scan revealed a large necrotic mass in the right cervicothoracic region, arising from the first rib and compressing the trunks of the brachial plexus and the subclavian artery, causing collapse of the innominate venous trunk and intraspinal extension from C7 to T1. An ultrasound-guided percutaneous biopsy established the diagnosis of Ewing sarcoma (monomorphic round blue cells, CD99+, FLI-1+) and was genetically confirmed by detection of an EWSR1 gene rearrangement via fluorescence in situ hybridization (FISH). Systemic staging evaluation revealed no distant metastases. The onset of thoracic outlet syndrome in a very young child requires immediate consideration of a malignant apico-thoracic tumor as a possible cause. Multidisciplinary pediatric management combining early induction chemotherapy and an appropriate local control strategy is essential for achieving recurrence-free survival while minimizing irreversible functional sequelae.